Many businesses, manufacturing companies, homeowners, etc. use generators in addition to power delivered by the local electric power company (also referred to as an electric power system or utility grid). However, non-utility owned generator systems connected to an electric power system create both operational and maintenance problems.
The operational problems include a non-utility owned generator system not being synchronized with a de-energized power grid included in the electric power system. The lack of synchronism between the non-utility owned generator system and the electric power system cause a higher than normal voltage across open isolation devices included between the non-utility owned generator and the electric power system, as well as higher than normal current flow when the isolation devices are closed. The higher than normal voltages across the opened isolation devices damages the devices and the higher than normal current flows tend to prematurely open over-current protection devices associated with the non-utility owned generator system and the electric power system.
Maintenance problems include personnel inadvertently contacting portions of the electric power system which are energized from the non-utility owned generator system. This is a severe problem which often results in injury or even death. For example, during a severe winter storm, utility companies have to dispatch emergency crews throughout neighborhoods to repair downed transmission lines, etc. To safely repair the downed lines, isolation devices corresponding to a power grid including the downed transmission lines are intentionally opened so the downed lines and associated transmission components are de-energized. However, if a homeowner plugs in a portable generator system to provide electricity for his family, for example, the power from the portable generator system may be fed back into the de-energized power lines, transformers etc, critically injuring a maintenance worker.
To avoid these types of possibly fatal injuries, the utility company would have to manually isolate all portable generator systems, etc., connected to the local grid requiring maintenance so as to bring the voltage to a safe level before beginning any maintenance work. This is extremely ineffective and time-consuming. In fact, it is virtually impossible to know when and where every portable generator system will be used. Thus, the power companies require generator systems connected to the utility grid to include protective devices.